var num = document.getElementById("num")
var pass = document.getElementById("pass")
var label1 = document.getElementById("label1")
num.onfocus = function(){
    this.placeholder = ""
    
}
num.onblur = function(){
    if(this.placeholder != "用户名" || this.placeholder != "")
    this.placeholder = "用户名"
}
pass.onfocus = function(){
    this.placeholder = ""
    
}
pass.onblur = function(){
    if(this.placeholder != "密码" || this.placeholder != "")
    this.placeholder = "密码"
}
var lab = document.getElementById("Lable1");
